In an endoscope used for medical treatment, a sleek design of the insertion tube section of the endoscope and a reduction in length of a rigid portion at the distal end of the insertion tube section are demanded in order to improve its operation property. It is thus indispensable that the objective lens used in the endoscope is constructed so that aberration is completely corrected in practical use, a lens contour is small, and the entire length is reduced.
Conventional endoscope objective lenses, each having a simple arrangement of three lens elements, are set forth, for example, in Patent References 1 and 2 described below. In the former, chromatic aberration is favorably corrected although the arrangement is simple. In the latter, a compact design is executed and telecentricity is good.
Patent Reference 1: Japanese Patent Kokai No. Hei 5-107470
Patent Reference 2: Japanese Patent Kokai No. Hei 10-170821
An objective lens including four lens elements, in which a field lens is placed in the proximity of an image plane, is set forth, for example, in Patent Reference 3 described below. This objective lens has better telecentricity than in Patent References 1 and 2 and is suitable for a fiberscope, for instance.
Patent Reference 3: Japanese Patent Kokai No. Hei 2-176611